This invention relates generally to magnetic flowmeters, and more particularly to an excitation circuit for a magnetic flowmeter whose electromagnet is excited by a low-frequency excitation current.
In a magnetic flowmeter, an electromagnetic field is established therein by an electromagnet having an excitation coil, the field being intercepted by a fluid passing through the flow tube to induce a flow rate signal in a pair of diametrically-opposed electrodes. Magnetic flowmeters are known which make use of a low-frequency excitation current, such as one whose excitation frequency is well below that of the commercial power-line frequency. One can, by means of a low-frequency excitation type magnetic flowmeter, obtain a flow rate signal having an excellent signal-to-noise ratio; for unwanted magnetic coupling and/or electrostatic coupling between the excitation coil and the signal lead wires connected to the flow tube electrodes may be reduced considerably.
In order to eliminate unwanted fluctuations from the flow rate signal yielded by a low-frequency excitation type magnetic flowmeter, which result from fluctuations in the excitation current for the electromagnet, the general practice is to provide a reference voltage-detecting circuit. This circuit is arranged to produce a reference voltage proportional to the excitation current, the ratio of this reference voltage to the flow rate signal being determined by means of a converter or divider.
In the copending application of Kazuie Suzuki, entitled "Magnetic Flowmeter," Ser. No. 888,631, filed Mar. 21, 1978, there is disclosed a reference voltage-detecting circuit which makes use of a current transformer interposed between the excitation current source and the excitation coil, the arrangement being such as to produce a reference voltage having a waveform identical to that of the excitation current without distortion despite saturation of the transformer core.
In this prior arrangement, the current transformer for detecting the reference voltage is interposed between the excitation coil of the magnetic flowmeter and the excitation current source. The primary of the transformer is constituted by a pair of primary windings in an arrangement wherein the excitation current is caused to flow in one direction in the first primary winding and a current produced by the counter electromotive force induced in the excitation coil in every on-off transient of the excitation current is caused to flow in the opposite direction in the second primary winding. A reference voltage a-c signal changing in both positive and negative directions is obtained from a secondary winding of the transformer. By "full-wave" rectifying this reference voltage a-c signal, a reference voltage which has a waveform similar to that of the excitation current is obtained. This reference voltage as well as the signal obtained from the flowmeter is applied to a divider which yields an output signal proportional to flow rate, the signal being substantially independent of fluctuations in the excitation current. However, this prior arrangement is subject to errors in the reference voltage signal which results from the temporary decrease in its level in the region where the a-c electric power crosses the zero line.